Various kinds of silver halide photographic materials are now commercially sold and various methods for forming images with the materials are known and are utilized in various fields. The halogen composition constituting the silver halide emulsions used in these many photographic materials is mostly a silver iodobromide, silver chloroiodobromide or silver chlorobromide consisting essentially of silver bromide for the purpose of attaining the high sensitivity.
On the other hand, in the products, such as photographic materials for color printing papers, to be used in the commercial field where an extremely large amount of prints are required to be finished in a short period of time, a substantially silver iodide-free silver bromide or silver chlorobromide is used because of the necessity of accelerating the development speed.
Recently, there has been a need for improving the rapid processability of color printing photographic materials, and various studies have been made thereon. It is well known that by increasing the silver chloride content in the silver halide emulsion to be used in the photographic materials, a remarkable improvement of the developability (rapid development speed) of the materials can be obtained.
However, a silver halide emulsion with a high silver chloride content is known to have some defects in that it is easily fogged, it cannot be given a high sensitivity by conventional chemical sensitization, and it frequently has a reciprocal law failure which means that it shows a large variation of sensitivity and gradation in accordance with exposure intensity.
In order to overcome the above-mentioned defects in the silver halide emulsions with a high silver chloride content, various techniques have been proposed and illustrated.
JP-A-58-95736, JP-A-58-108533 (U.S. Pat. No. 4,564,591) JP-A-60-222845 (U.S. Pat. No. 4,605,610) (the term "JP-A" as used herein refers to a "published unexamined Japanese patent application") mention that various silver halide grain structures having a high silver bromide content layer are effective for overcoming the defects of silver halide emulsions with a high silver chloride content. Introduction of the high silver bromide content layer surely causes various variation of the photographic properties of the resulting silver halide emulsion with a high silver chloride content. However, the effect of improving the reciprocal law failure was only slight even by the above technique.
JP-A-51-139323 and JP-A-59-171947 and British Patent 2,109,576A mention that incorporation of Group VIII metal compounds is effective for elevating the photographic sensitivity and for reducing the reciprocal law failure characteristic. JP-B-49-33781 (the term "JP-B" as used herein refers to an "examined Japanese patent publication"), JP-A-50-23618, JP-A-52-18310, JP-A58-15952, JP-A-59-214028 and JP-A-61-67845, German Patent 2,226,877, German Patent OLS 2,708,466 and U.S. Pat. No. 3,703,584 mention that incorporation of rhodium compounds or iridium compounds is effective for elevating the high contrast and for reducing the reciprocal law failure characteristic. However, these techniques are still insufficient for overcoming the problems in the high silver chloride content silver halide emulsions for use in the present invention. JP-A-62-75436 and JP-A-62-80640 mention use of rhodium compounds for obtaining low sensitive photographic materials capable of being processed in a daylight room. U.S. Pat. No. 3,703,589 mentions use of the above metals in direct positive type silver halide emulsions. JP-B-48-35373 mentions incorporation of water-soluble iron compounds into silver chloride emulsions obtained by a normal mixing method to give a high contrast black-and-white printing photographic materials at a low cost. However, all of these photographic materials were still insufficient in sensitivity, reciprocal law failure characteristic and latent image stability, and the problems in the prior art could not still be overcome up to the present.